(1) Field of the Invention
The present invention generally relates to an image data coding apparatus, and more particularly to an image data coding apparatus in which binary images, such as character images and halftone dot images, are coded. The present invention can be applied to facsimile machines and image filing systems.
(2) Description of related art
Conventionally, binary images, such as character and line images, are generally coded in accordance with an MH (modified Huffman) coding scheme, an MR (modified READ) coding scheme or the like. However, pseudo-multilevel images, such as halftone dot images and dither images, can not be coded at a high compression rate by the above coding schemes. That is, as the MH and MR coding schemes are suitable for coding images having white dots or black dots continuously arranged in a row, in a case where images having dispersed black dots and white dots are coded in accordance with the MH or MR coding scheme, the coding efficiency is deteriorated.
Conventionally, a predictive coding method has been known as a coding method suitable for effectively coding pseudo-multilevel images. In the predictive coding method, some dots strongly correlated with an objective dot are selected from scanned dots, a value of the objective dot is predicted based on the selected dots and a predicted value of the objective dot is obtained. Then, a difference between the predicted value and an actual value of the objective dot is coded. In addition, a predictive component coding method can be also applied to the pseudo-multilevel images, the coding method being obtained, from a viewpoint of an information theory, by introducing a concept from a source component coding method into the predictive coding method.
However, in the predictive coding method, it is necessary to use a predictive table corresponding to an image to be coded. Thus, a scope of images which can be coded in accordance with the predictive coding method is limited. Further, coding efficiency is not satisfactory.